1. Field of the Invention
The apparatus and method of the present invention relates generally to the reduction of overvoltage transients on the alternating current power signals produced by a conventional variable speed constant frequency (VSCF) power generation system after a load reduction or no-load condition occurs.
2. Description of the Prior Art
Variable speed constant frequency (VSCF) power generation systems are well-known, and such systems are being used in aircraft applications. Representative of conventional VSCF power generation systems are the systems disclosed in the following U.S. patents:
______________________________________ Re 26,630 Peaslee issued 7/15/69 3,152,297 Peaslee issued 10/6/64 3,289,070 Caldwell et al issued 11/29/66 3,320,514 Lawrence issued 5/16/67 3,400,321 LaFuze issued 9/3/68 3,419,785 LaFuze issued 12/31/68 3,593,106 LaFuze issued 7/13/71 3,641,418 LaFuze issued 2/8/72 3,745,471 LaFuze issued 7/10/73 3,873,928 LaFuze issued 3/25/75 3,902,073 LaFuze issued 8/26/75 3,908,161 Messenger issued 9/23/75 ______________________________________
An excellent generalized discussion of conventional VSCF systems is found in Aircraft Electrical Power Seminar--Technical Proceedings, General Electric Company, Aircraft Equipment Division, Binghamton, N.Y., May 10-11, 1977.
Conventional VSCF power generation systems produce overvoltage transients on the alternating current power signals provided by the cycloconverters for the first several cycles, e.g., 3 to 4 cycles, of the alternating current after a load reduction or no-load condition occurs. These overvoltage transients on the alternating current power signals cause several problems with conventional VSCF power generation system, including (1) miscommutations of the SCR's of the cycloconverters due to the decrease in commutating time resulting from the increased voltage level and the leading current drawn by the filter capacitors connected to the alternating current power output signals from the cycloconverters; (2) equipment malfunctions of the equipment loads connected to the alternating current power signals caused by the overvoltage transients on the alternating current power signals.
The overvoltage transients which appear for the first several cycles of the alternating current after a load reduction or no-load condition occurs in a conventional VSCF power generation system are due to two factors. Each cycloconverter in a conventional VSCF power generation system exhibits a source impedance, and must also be provided at its output with a capacitor having a high capacitance value, which acts as a low pass filter to remove the rectifier ripple components generated by the cycloconverters on the alternating current power signals. When a load reduction or no-load condition occurs, the voltage levels of the alternating current power signals increase above the voltage level specification because of the large leading current drawn by the filter capacitors and because of the source impedance of the cycloconverters. The individual phase voltage regulators of the conventional VSCF system are unable to reduce the voltage levels of the alternating current power signals within the first several cycles, e.g., 3 to 4 cycles, after the load reduction or no-load condition occurs because of the slowness of the voltage regulating control loop. The speed of the voltage regulating control loop cannot be increased in conventional VSCF power generation systems because of the loop instabilities that are introduced.
Thus, the voltage levels of the alternating current power signals provided by a conventional VSCF power generation system increase above the voltage level specification for the first several cycles of the alternating current power signals after a load reduction or no-load condition occurs. The overvoltage transients which occur for the first several cycles of the alternating current after a load reduction or no-load condition occurs are particularly troublesome with respect to commutation of the SCR's of the cycloconverter. Specifically, miscommutation of the SCR's substantially increases because of the reduction in available commutating time due to the voltage increase of the alternating current power signals and the large leading currents drawn by the filter capacitors connected to the outputs of the cycloconverters. Miscommutation problems could be reduced if the voltage level of the alternating current power during the first few cycles after a load reduction or no-load condition occurs could be reduced so as to provide the cycloconverters with more circuit turn-off time so as to provide the needed time to commutate naturally the SCR's. In this connection, it should be noted that it is more difficult to commutate a zero power factor load, such as the filter capacitors connected to the outputs of the cycloconverter, than it is to commutate a unity power factor load. In a no-load condition, the cycloconverters of a conventional VSCF power generation system elecrtrically see only the zero power factor load produced by the filter capacitors. Thus, for purposes of reducing miscommutations of the SCR's of the cycloconverters of a conventional VSCF power generation system, it would be advantageous to reduce the transient overvoltage increase of the alternating current power signals which occurs during the first several cycles after a load reduction or no-load condition occurs.